Technical FAQ: Electronic shifting, rim vs disc brakes, and Chloé Dygert's crash

Have a question for Lennard? Please email him at veloqna@comcast.net to be included in Technical FAQ.

Dear Lennard,
Something untypical: I want to rebuild my carbon MTB to a “Mullet”/“Frankenstein” bike.

I really love the +500 percent SRAM Eagle 1x groups, and I also love hydraulic disk brakes. And I really like the idea of the new sub brake lever Shimano BL-RX812 in combination with a drop handlebar/gravel bar.

All this together made me think of this evil Frankenstein plan and I’m not sure if this all will work…

1. I want to upgrade to the SRAM XX1 Eagle AXS Upgrade-Kit. The plan is to get two shifting levers; one is mounted on the flats and one I would modify to be connected with the drop bar shifters. Do you know if SRAM AXS supports a pairing with more than one shift lever? This would be essential for my plan!
2. I would choose Shimano GRX Di2 ST-RX815 brake levers because mainly of the hydraulic braking performance and the good handling and ergonomics. I know that Di2 is not compatible with AXS. I was still thinking of hacking the buttons of the GRX levers and resoldering with the second SRAM AXS shifting lever with small thin cables. I would hide the hacked lever somewhere inside the stem or the frame. Do you think the switch of the Di2 is electrically compatible with the AXS? I guess it should be a normal pulse micro switch button? I am not sure but in the Di2 system, there are always cables to the main unit? And for me — as an electronic engineer — I just need to find an electrical connection to the buttons.
3. The last stage is the hydraulic brake levers Shimano BL-RX812. These should be no problem since they are compatible with the ST-RX815. Really cool, two brake levers per brake.

With this setup, I should be able to have this setup: Drop handlebars with three different hand positions and access to brake levers and shifters, additional wireless MTB shifting range, and hydraulic brakes.

And of course still full suspension and fat tires. The ultimate setup with the best of two worlds. At least in my opinion and for my use.
— Stefan

Dear Stefan,
It will only work if you also use Shimano hydraulic brake calipers with the Shimano levers, since SRAM brakes use DOT fluid, and Shimano brakes use mineral oil.

Yes, SRAM eTap AXS allows multiple shift buttons. You can use SRAM Blips or SRAM MultiClics.

There is no need to do any soldering, nor is there even a need to purchase SRAM shifters. Instead, you just get a SRAM Blip Box and connect your Blips or MultiClics to it. You could have MultiClics or Blips on the tops of the bar and Blips near the brake levers. This would certainly be the simplest and quickest way to set it up.

If you would rather use the GRX Di2 lever buttons than Blips attached next to them, then you can probably create a pair of wires that will plug one shift button from each GRX lever into the Blip Box. You could snip the Blip off of its wire, snip one connector off of a Shimano Di2 E-Tube wire, and splice the two wires together. Then you could plug the E-Tube end into the lever and the other end with the SRAM plug into the Blip Box.

The shifters, whether on SRAM eTap or Shimano Di2, are just switches, and they will not care which system they are switching.
― Lennard

Dear Lennard,
From the slo-mo of the Chloé Dygert crash, it looks like at the start of the wobbles she was lifting off the saddle (or maybe other dynamic forces were forcing that). Could that have raised her center of gravity enough, or removed stability to make the whole bike unstable and uncontrollable once she started skidding? Sad, regardless.
—Larry

With disc brakes distributing the braking force nearer to the axle, I’m wondering if the same physics apply?
— Randy

Dear Randy,
The same physics applies to disc brakes. Whether the brakes are applied at the rim or on a brake rotor, the deceleration of the wheel and the resultant change in angular momentum is the same.
― Lennard

Dear Lennard,
I’m wondering how tire PSI corresponds to pounds of rider weight. I know what PSI stands for, and I know how to determine the optimal PSI for my rides. But I’m wondering how tire PSI corresponds to the number of pounds of weight my body puts on the tire. For flattery, let’s say I weigh 180 pounds. For convenience, say half of that is on the front wheel, and the contact patch of the tire is one square inch. Why don’t I need at least 90 PSI in the front tire to hold up the front half of my weight, but I can use considerably less than that? Or, is the 90 pounds supported by the total number of square inches in the front tire? I know when I’m moving my weight has more effect than just sitting still on my bike, but I don’t know how to calculate this or how to measure the number of square inches there are in my front tire. If I did maybe I could figure out the answer to what I’m wondering about.
— Ray

Dear Ray,
No, the 90 pounds you are talking about is not supported by the total number of square inches of the tire. Rather, it is supported by the area (the total number of square inches of) the tire contact patch on the ground. The more weight on the wheel, the larger that contact patch will be at a given tire pressure. Perhaps now you can see why it makes no sense to say that the contact patch is equal to one inch and then talk about varying the air pressure. The contact patch’s area is dependent on air pressure as well as weight (load) on the wheel; its area will decrease with increasing air pressure under a given load and increase with increasing load at a given air pressure.

So, there is no need to measure the “number of square inches on the front tire.” Measuring the tire contact patch can be done by loading the front wheel while the tire is sitting on a glass table. Crawl underneath the table and trace around the elliptical contact patch with a marker. Measure the length and width of the ellipse you have traced. Its area will be equal to half of its length times half of its width times pi.
― Lennard

Lennard Zinn, our longtime technical writer, joined VeloNews in 1987. He is also a custom frame builder (www.zinncycles.com) and purveyor of non-custom huge bikes (bikeclydesdale.com), a former U.S. national team rider, co-author of “The Haywire Heart,” and author of many bicycle books including “Zinn and the Art of Road Bike Maintenance,” “DVD, as well as “Zinn and the Art of Triathlon Bikes” and “Zinn’s Cycling Primer: Maintenance Tips and Skill Building for Cyclists.” He holds a bachelor’s in physics from Colorado College.

Follow @lennardzinn on Twitter.